Multi-speed bicycles typically have drive trains comprising a chain which passes around one sprocket in each of a front set of sprockets and a rear set of sprockets. A derailleur mechanism allows a rider to selectively move the chain to a different one of the sprockets in each set of sprockets. This enables the rider to select a gear ratio which is most appropriate for the current riding conditions.
One disadvantage of derailleur type transmissions is that it is only possible to increase the number of gear ratios offered by increasing the number of sprockets in one or both of the sprocket sets. The result is that the chain often extends at an angle between the front and rear sprocket sets. This is undesirable because it reduces the efficiency of the chain drive and places undesirable stresses on the entire drive train.
Another disadvantage of currently available derailleur systems is that they do not always shift smoothly under load, for example, when a rider is riding a bicycle up a hill. A further disadvantage of some derailleur type multi-speed transmissions is that a significant amount of force is generally required to shift the chain between adjacent sprockets in one of the sets of sprockets. Therefore, if it is desired to automate the shifting function by controlling shifting with a computer controlled actuator, for example, the actuator must be capable of supplying the necessary force. Consequently, an expensive and typically power hungry actuator is required. The result is that electronically controlled automatic derailleur type transmissions have not been widely accepted.
There have been many suggestions of alternative variable ratio transmissions suitable for use in bicycles. Many such transmissions include a "variable diameter sprocket." In these transmissions, one of the front and rear groups of sprockets is replaced with a segmented sprocket. The segmented sprocket has a number of radially movable segments which engage the chain. By moving the segments inwardly or outwardly, the effective diameter of the segmented sprocket can be changed, thereby varying the gear ratio of the transmission.
Some examples of transmissions which employ a segmented sprocket are shown in Hufschmidt, U.S. Pat. No. 4,634,406; Gummeringer, U.S. Pat. No. 4,696,662; Pritchard, U.S. Pat. No. 4,797,879; Husted, U.S. Pat. No. 4,810,235; Schendel, U.S. Pat. No. 5,476,422; Pike, U.S. Pat. No. 4,740,190; Husted, U.S. Pat. No. 4,645,475; Rathert, U.S. Pat. No. 4,516,960; Leonard, U.S. Pat. No. 4,030,373; Leonard, U.S. Pat. No. 4,816,008; Newell, U.S. Pat. No. 3,995,508; Dutil et al., U.S. Pat. No. 4,598,920; Chappel, U.S. Pat. No. 4,836,046; Japanese Patent Disclosure No. 55-86948; and Chilcote et al., U.S. Pat. No. 4,850,039. These variable sprocket transmissions use various schemes to cause the sprocket segments to move inwardly or outwardly to vary the drive ratio provided by the transmission. Most of these transmissions have not been commercially acceptable, either because they are inordinately complicated, too heavy, insufficiently robust, inefficient, or are not well suited to automation with low powered actuators. Some prior art transmissions of this general type select gear ratios automatically and do not permit a user to select a ratio different from the automatically selected ratio manually. Currently available multi-ratio chain drive transmissions for non-bicycling applications have similar limitations.
What is needed is a reasonably simple variable ratio transmission which is reasonably light in weight, yet reasonably robust, reasonably inexpensive to fabricate and readily shifted from one ratio to another.